Microbial Processes and Down-Hole Mesocosms in Two Anaerobic Fractured-Rock Aquifers

By Don A. Vroblesky, Paul M. Bradley, and John F. Robertson

ABSTRACT

Aqueous chemistry, hydrogen-gas measurements, and downhole mesocosms were
used to characterize the microbial terminal electron-accepting processes in
anaerobic fractured-rock aquifers at Montville, Connecticut, and at the Mirror
Lake research site, New Hampshire, and to examine the potential for the microbial
community to degrade a target contaminant, toluene. At the most contaminated
horizon in the petroleum-hydrocarbon-contaminated aquifer at Montville, the
terminal electron-accepting process was methanogenesis. At shallower horizons,
sulfate- or iron-reduction prevailed. The presence of organic carbon allowed
similar depletion of oxygen in ground water at wells CO-11 and RR-1 (Mirror
Lake site). At well CO-11, the advective transport of sulfate apparently was
rapid enough or sufficient amounts of oxidized iron were available to prevent
methanogenic conditions from developing. At well RR-1, however, the lack of
more efficient electron acceptors and the increased availability of organic
carbon resulted in methanogenic conditions. Downhole mesocosms placed next
to fractures in the aquifer were colonized by native bacteria. The mesocosms
provided a sampling approach to recover native bacteria and to use the recovered
bacteria to examine the potential for contaminant biodegradation.